1. Field of the Invention
The present invention relates generally to a method for depositing a superconducting film of niobium nitride (NbN) without substrate heating and more particularly to depositing a NbN film with a superconducting transition temperature (T.sub.c) comparable to the T.sub.c of bulk NbN.
2. Description of the Prior Art
A Josephson junction tunnel device is a multilayer structure including two electrodes separated by a barrier. There has been considerable interest in the fabrication of Josephson tunnel devices using refractory niobium nitride (NbN) thin film electrodes. This electrode material has several pronounced advantages compared to other materials that have been used previously: high T.sub.c, stable B1 crystal structure, resistance to radiation damage and ease of preparation. The quantity T.sub.c is the temperature below which a material is superconductive. T.sub.c is not actually a single value but is characterized by a transition width, .DELTA.T.sub.c. It is generally desirable to maximize T.sub.c in order to minimize cooling requirements.
Typically, these high T.sub.c NbN films are reactively sputtered onto a heated substrate by a d.c., r.f. or magnetron sputtering technique utilizing a niobium target cathode in an argon/nitrogen atmosphere. When the substrate is maintained at or near ambient temp during the growth of the films it is observed that Nb-N compounds are formed which have superconducting transition temperatures which are usually below 10.degree. K. with the highest values approaching 12.degree. K. In order to obtain thin deposited films with T.sub.c approaching 16.degree. K., the value for bulk NbN, the substrate has to be heated to temperatures above 700.degree. C. which assists in the formation of NbN compounds with suitable crystallographic structure and composition ratios which yield T.sub.c near the bulk value.
The high T.sub.c films formed utilizing heated substrates have been found to contain approximately 4% carbon as determined by Auger analysis. This presence of carbon is believed to account for the high T.sub.c of these films.
It is advantageous to obtain electrodes with a high T.sub.c in order to reduce the need to maintain the Josephson junction tunneling device at extremely low temperatures. However, the substrate heating required by the existing fabrication methods described above degrades the preformance of the device.
Thus, when existing electrode fabrication methods are used, either a low T.sub.c film may be deposited without substrate heating or a high T.sub.c film may be deposited with substrate heating and the attendant disadvantages thereof.